Relationships between needle retention, growth and climate of Pinus sylvestris in Valais

The assessment of the crown transparency of trees is a key component of long-term monitoring programmes of forest health. However, the assessments are subjective and many problems exist with the interpretation of the resulting data. In Switzerland, there has been a marked trend for increasing crown transparency over the last 10 years, but the extent to which this represents a new phenomenon is unknown. For example, it has been argued that trees have always had transparent crowns and the trend seen in the period 1985-1996 simply represents a natural fluctuation.

Project aims

The aim of this project is to use the needle retention assessment method developed in Finland to examine long-term needle retention of Scots pine (Pinus sylvestris) in Valais, Switzerland. This region was chosen because the presence of fluorine pollution in the 1970s should be reflected in the needle retention scores, providing evidence that the method can be used to reconstruct the impact of pollutants. The data will be related to growth information from the trees, and the data used in a modelling study of needle retention - growth - climate relationships.

Specific questions being addressed in the project are:

How has needle retention in Scots pine and other pine species changed over the last 100 years in the Swiss Alps?

To what extent has climate and extreme weather conditions determined needle retention patterns in pines?

Are the data on crown condition from Switzerland (1984-96) supported by needle retention statistics?

Can needle retention analysis, combined with year ring analysis, reveal anything about the decline of Pinus sylvestris in stands near Visp, Canton Valais?

5 trees each from 8 sites in central Valais (between Visp and Sierre) have been
felled (Photo 1, 64 k) and segmented (Photo
2, 128 k). The analysis method is based on the examination of the traces
(Photo 3, 64 k) left in the wood from the vascular
system of a tree connecting the short shoots (Photo
4, 96 k) to the pith of the main shoot. These traces reach from the
pith to that year when the needles were shed, and can be seen clearly as
dark spots when the wood is planed longitudinally. Needle traces can be
counted from both the main stem and branches, although this study is concentrating
on the main stem. When the counting of needle traces per year ring is repeated
for successive years, the long-term chronologies for annual needle retention,
needle age, needle budget and needle retention patterns can be produced.
The method is described in Jalkanen et al. (1994, see below).